Gold-semiconductor phase change memory for archival data storage

ABSTRACT

A structure for storing digital data is provided, with a high reflectance layer comprising a gold film formed over a semiconductor layer, and a plurality of low reflectance portions comprising a mixture of a gold material and a semiconductor material. The plurality of low reflectance portions have top surfaces comprising more semiconductor material than the gold material. The invention also provides a method of changing reflectance on a data storage disk, comprising irradiating a laser light beam onto a gold film formed over a semiconductor layer, and raising the temperature of the gold film above a eutectic temperature for a mixture of gold and the semiconductor layer.

FIELD OF THE INVENTION

The invention relates generally to archival data storage on optical disks, and more particularly to using novel materials that provide greater long-term stability to the stored data.

BACKGROUND OF THE INVENTION

Personal computer users, businesses, and public offices are faced with a deluge of data in the form of digital information. The question of how to preserve this data for the next decade, and for the ages, has yet to be answered. The most common data backup includes storing the data on a writable compact disk CD, CD-R (compact disk, write once read-only memory), CD-ROM (compact disk, read-only-memory), or a CD-RW, short for CD-ReWritable disk. A CD-ROM is an adaptation of the CD and is designed to store data in the form of text and graphics, as well as sound. A CD-RW is a type of CD disk that enables a user to write onto the disk multiple times. A CD-R comprises an organic layer sandwiched between a transparent base and a reflective layer. When heated by a focused laser, the dye layer melts and forms a series of pits, which are readable by a laser beam as 0's and 1's.

The technology behind a CD-RW is known as optical phase-change, an optical storage technology in which data is written with a laser that changes dots on the disk between amorphous and crystalline states. Phase change is a type of CD recording technology that enables the disks to be written, erased, and rewritten through the use of a layer of a special material for the recording layer—the phase change layer—that can be changed repeatedly from an amorphous (formless) to a crystalline state. The crystalline areas allow the metalized layer to reflect the laser beam better, while the non-crystalline portion absorbs the laser beam, so the beam is not reflected. An optical head reads data by detecting the difference in reflected light from amorphous and crystalline dots.

During writing, a focused laser beam selectively heats areas of the phase-change material above the melting temperature, so all the atoms in this area can rapidly rearrange. The recording phase-change layer is sandwiched between dielectric layers that draw excess heat from the phase-change layer during the writing process. Then, if cooled sufficiently quickly, the random state is “frozen-in,” and the so-called amorphous state is obtained. The amorphous version of the material has different reflection properties the laser dot was written, resulting in a recognizable CD surface. Writing takes place in a single pass of the focused laser beam, which is referred to as “direct overwriting,” and can be repeated several thousand times per disk. Once the data has been burned, the amorphous areas reflect less light, enabling a “Read Power” laser beam to detect the difference between the lands and the pits on the disk. The recorded tracks on a CD-RW disk are read in the same way as regular CD tracks. That is, by detecting transitions between low and high reflectance, and measuring the length of the periods between the transitions. The only difference is that the reflectance is lower than for regular CDs.

A digital versatile disk (DVD) provides an optical disk technology that allows for much greater storage as compared with CDs. With reference to FIGS. 1 and 2, a DVD's sevenfold increase in-data capacity over the CD has been largely achieved by tightening the tolerances throughout the predecessor CD system. The tracks on the DVD are placed closer together, thereby allowing more tracks per disk than found on CDs. As shown in FIG. 2, the DVD track pitch 4 is reduced to 0.74 microns, less than half of CD's 1.6 micron track pitch 2, as shown in FIG. 1. The pits 6, in which the data is stored, are also a lot smaller, allowing more pits per track. The minimum pit length 10 of a single layer DVD is 0.4 microns, as compared to 0.83 microns pit length 8 for a CD. With the number of pits having a direct bearing on capacity levels, the DVD's reduced track pitch and pit size alone give DVD ROM disks four times the storage capacity of CDs. The packing of as many pits as possible onto a disk is, however, the simple part. The real technological breakthrough of the DVD was with its laser. Smaller pits mean that the laser has to produce a smaller spot, and the DVD achieves this by reducing the laser's wavelength from the 780 nanometers infrared light of a standard CD, to 635 nm or 650 nm red light.

The first-generation CD players used a 780 nm AlGaAs laser diode developed in the early 1980s. With this technology, a CD-ROM stored about 650 Mbytes of information. The shortest wavelength commercially-viable device that was made in this system was about 750 nm. Further shortening of the wavelength called for a different material, and in the late 1980s red-emitting laser diodes were developed in the AlGalnP system, grown lattice-matched on a GaAs substrate. This material has provided the laser for new DVDs, which store about 4.7 Gbytes of information. Different materials are used to make a laser emit blue light, e.g., at wavelengths in the range of 430 nm to 480 nm. One technique reported has been laser action at 77K from a GaN-based device by researchers at Nichia Chemical Industries in Japan. Nichia announced pulsed room temperature operation at the end of 1995, and continuous operation in early 1997. By August 1997 the room temperature operating life had reached 300 hours. Based on accelerated life-testing at elevated temperatures, Nichia reported in 1999 a room temperature operating life of about 10000 hours at room temperature. A wide variety of solid state laser diodes are now available for use in CD-ROM or CD-ROM like technology.

While current optical disk technologies such as DVD, DVD±R, DVD±RW, and DVD-RAM use a red laser to read and write data, a new format uses a blue-violet laser, sometimes referred to as Blu-ray. The benefit of using a blue-violet laser (405 nm) is that it has a shorter wavelength than a red laser (650 nm), which makes it possible to focus the laser spot with even greater precision. This allows data to be packed more tightly and stored in less space, so it is possible to fit more data on the disk even though it is the same size as a CD or DVD. This together with the change of numerical aperture to 0.85 is what enables Blu-ray Disks to hold 25 GB. Blu-ray technology should become available in the 2005 to 2006 time frame. Some new techniques proposed for archival storage have included “a polymer/semiconductor write-once read-many-times memory” and some “novel concepts for mass storage of archival data” using energetic beams of heavy ions to produce radiation damage in thin layers of insulators.

Current CD-ROM memories based on changes in organic dyes or phase changes in layers may degrade over time and become unreadable. Although at normal temperature and humidity the life span of CD could be in excess of 100 years, the life span of data on a CD recorded with a CD burner could be as little as five years if it is exposed to extremes in humidity or temperature. And, if an unprotected CD is scratched it can become unusable. What is needed is a disk that can provide greater long-term stability for the stored data.

BRIEF SUMMARY OF THE INVENTION

The invention relates to a writable optical data storage medium with good long term stability. The data storage medium, which can be a disk, for example, comprises a substrate having a dielectric layer formed thereon, a semiconductor layer formed over the dielectric layer, and a gold containing film formed over the semiconductor layer. A protective layer may be formed over the gold containing layer.

Data can be written onto the medium by a laser which causes formation of a mixed material portion in the gold containing film and the semiconductor layer. The mixed material portions of the medium have a lower reflectivity than other portions of the medium having the undisturbed gold containing film, enabling the medium to be read.

The invention also relates to a system for writing data and reading data from an optical data storage medium. The system comprises a device capable of irradiating a laser beam onto a medium which has a substrate with a first dielectric layer, a semiconductor layer formed over the first dielectric layer, and a gold containing film formed over the semiconductor layer. The invention provides a writing laser beam capable of forming mixed material portions on the medium which contain both gold and semiconductor material. The system also provides a reading laser beam which can read, but otherwise not write data onto the medium.

The invention also relates to a method of changing reflectance of selected areas on a data storage medium, comprising irradiating a laser light beam onto a gold film formed over a semiconductor layer to raise the temperature of the gold film above a eutectic temperature causing the creation of a mixture of gold and the semiconductor layer. The invention also provides a recorded optical medium which has a support structure, a first material layer formed over the support structure, and a second light reflective material layer formed over the first material layer. The second light reflective material has a first light reflectance property. The first and second material layers have a property such that a light beam applied to a region of the second material layer heats the first and second material layers and causes a mixture of materials from the first and second material layers, and also causes a second light reflectance property for the region which is different from the first light reflectance property.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other features of the present invention will be apparent from the following detailed description and drawings which illustrate preferred embodiments of the invention, in which:

FIG. 1 shows data storage characteristics of a typical CD;

FIG. 2 shows data storage characteristics of a typical DVD;

FIG. 3 is a graph showing optical properties of gold;

FIG. 4 is a perspective view of a data storage medium in accordance with an embodiment of the invention;

FIG. 4A is a perspective view of a data storage medium in accordance with another embodiment of the invention;

FIG. 4B is a perspective view of a data storage medium in accordance with another embodiment of the invention;

FIG. 5 is a partial side-sectional view of the data storage medium of FIG. 4 during a write operation;

FIG. 6 is a partial side-sectional view of the data storage medium of FIG. 4 during a write operation;

FIG. 7 shows an equivalent electrical circuit for the data storage medium of FIG. 4;

FIG. 8 is a graph of temperature versus time during a write operation;

FIG. 9 is a phase diagram of gold and silicon;

FIG. 10 is a phase diagram of gold and germanium;

FIG. 11 shows an equivalent electrical circuit for the data storage medium of FIG. 4;

FIG. 12 shows a partial representation of a layer of the data storage medium of FIG. 4;

FIG. 13 shows a partial equivalent electrical circuit of the circuit of FIG. 11;

FIG. 14 is a graph of temperature versus time during cooling;

FIG. 15 shows a partial side-sectional view of the data storage medium of FIG. 4 after a write operation;

FIG. 16 shows a partial side-sectional view of the data storage medium of FIG. 4 after a write operation;

FIG. 17 shows a system for using the data storage medium of FIG. 4; and

FIG. 18 shows a block diagram of a device for writing and reading from the data storage medium device of FIG. 4.

DETAILED DESCRIPTION OF THE INVENTION

In the following detailed description, reference is made to the accompanying Figures, which form a part hereof and show by way of illustration specific embodiments in which the invention may be practiced. These embodiments are described in sufficient detail to enable those skilled in the art to practice the invention, and it is to be understood that other embodiments may be utilized, and that structural, logical, and other changes may be made without departing from the spirit and scope of the present invention.

The invention relates to an archival storage medium, and is based on a reflectance change between a reflective metal portion of the medium and a mixed metal material and semiconductor material portion of the medium. In an exemplary embodiment, the medium is an optical disk and the reflective metal is gold. Gold has desirable properties for long term archival stability, for it combines good tarnish resistance with consistently high reflectance throughout the near, middle, and far infrared light wavelengths. In FIG. 3, a graph of pure gold's reflectance versus wavelength in nanometers is shown. Pure gold provides over 96% average reflectance from 650 nm to 1700 nm, and over 98% average reflectance 2000 nm to 1600 nm. In addition to its good optical properties, gold is also effective in controlling thermal radiation.

With reference to FIG. 4, an archival storage medium in the form of a memory disk 50 according to an exemplary embodiment of the invention is shown having a carrier wafer or substrate 60, over which is deposited a dielectric layer 58, for example an oxide layer. Although the invention is illustrated in the form of a disk in the exemplary embodiment, it may take other forms as well, for example, an optical card or other arrangement. The carrier wafer or substrate 60 can comprise a polycarbonate material, and the dielectric layer 58 can comprise a silicon dioxide layer, for example, or another dielectric layer. A semiconductor layer 56 is formed over the dielectric layer 58. The semiconductor layer 56 can comprise, for example, a silicon layer, a germanium layer or other semiconductor. In accordance with the invention, a thin gold film 54 is deposited, by evaporation, for example, onto the underlying semiconductor layer 56. Conventional techniques may be used to form the layers and films, and are well known to one skilled in the art. The gold film 54 is thereafter preferably covered with a protective transparent layer 52. The protective layer 52 can be a dielectric layer, for example, an oxide layer, which can comprise a silicon dioxide layer. The protective layer 52 can be another passivating material which has an adequate degree of transmission for the wavelength of a laser beam chosen to write data. In a preferred embodiment, the protective layer 52 is a transparent oxide layer 52 is transparent so that the gold film 54 retains its mirror-like reflective properties. A protected mirror-like gold film 54 combines the natural optical and spectral performance of gold together with the durability of hard dielectrics. In addition, coated gold films can be cleaned regularly using standard organic solvents, such as alcohol or acetone.

Referring again to FIG. 4, the gold film 54 can be deposited to a thickness of approximately a few 100 Å. In a preferred embodiment, the gold film 54 has a thickness of approximately 50 Å to approximately 300 Å. The thickness of the underlying semiconductor layer 56 is preferably several times greater than the thickness of the gold film 54. In a preferred embodiment, the thickness of the semiconductor layer 56 is approximately 200 Å to approximately 2000 Å. The thickness of the protective dielectric layer 52 is approximately 300 Å to approximately 1000 Å.

The archival storage medium according to the invention can have other constructions. For example, a recordable CD-R is typically formed with a recording layer next to the substrate. With reference to FIG. 4A, such a storage medium 250 has a substrate 260 on top of a gold film 254. The substrate 260 can comprise a polycarbonate material. A thin adhesive layer (not shown), chromium, for example, may be deposited between the substrate 260 and the gold film 254 to promote adhesion of the gold film 254 to the substrate 260, or a surface of the gold film 254 may be activated with a plasma to promote adhesion. A semiconductor layer 256 is disposed underneath the gold film 254. The storage medium 250 also comprises a dielectric layer 258 and a label layer 270. Alternatively, the storage medium can be a double-sided storage medium, as shown in FIG. 4B, wherein the storage medium 252 is recordable on both sides.

As discussed above, data is written to the data storage medium, e.g., a storage disk, by using a laser beam. With reference to FIG. 5, a laser beam 70 irradiated onto disk 50 passes through the protective layer 52, and heats the gold film 54 and the underlying semiconductor layer 56. In the embodiments of FIGS. 4A and 4B, the irradiated laser beam 70 passes through the substrate 260 and heats the gold film 254 and the underlying semiconductor layer 256. Hereinafter, for the sake of brevity, the invention is described with reference to the embodiment of FIG. 4 although it is similarly applicable to the embodiments of FIGS. 4A and 4B. The light of the laser beam 70 has a wavelength such that it is not significantly absorbed by the protective layer 52 so the laser beam 70 maintains its energy as it passes through the protective layer 52, and reaches the gold film 54. The gold film 54 absorbs the energy from the laser beam 70, and the heat from the laser beam 70 causes the gold film 54 to melt and diffuse into the underlying semiconductor layer 56.

As a result, with reference to FIG. 6, a molten region 72 is formed. The molten region 72 comprises a mixture of materials from the melted gold film 54 and semiconductor layer 56. The top surface 74 of the molten region 72 comprises a mixture of materials from the gold film 54 and semiconductor layer 56.

After exposure of the laser beam 70 onto the molten region 72 is stopped, the molten region 72 cools and becomes a solid mixture region comprising materials from the gold film 54 and semiconductor layer 56. The solid mixture regions are not as reflective as areas of the mirror-like gold film 54 that were not melted. The difference in reflectance can be detected by a laser during a read operation which has a lower energy than the laser that writes data onto the data storage medium. It has been found by the inventors that for maximum change in reflectance, which is desirable for data storage, the molten region 72 should contain more semiconductor layer 56 material than gold film 54 material. This is achieved by forming the semiconductor layer 56 thicker than the gold film 54, as discussed above, and by supplying energy sufficient to melt not only the gold film 54, but also the underlying semiconductor layer 56, as discussed below.

An exemplary laser for the above-described write operation is a blue laser, having a wavelength of 405 nm to 480 nm. Such a laser works best for highest density data storage. The blue laser produces more transmittance and absorption of the light (or not as much reflectance) than other lasers when irradiated onto the gold film 54. But, the blue laser produces most absorption of the light by the semiconductor layer 56, which is desirable because it allows for low reflectance from surface regions comprising, at least in part, the semiconductor layer 56 and the gold film 54.

Thermal design considerations of the disk 50 are now discussed. During writing of data to the disk 50, the energy of the laser beam 70 is absorbed by a small area of the gold film 54 and underlying semiconductor layer 56, thereby producing the molten region 72 as discussed above. The molten region 72 is mostly a semiconductor material, such as silicon, germanium or other semiconductor. The range of the laser beam's high temperature penetration into the disk 50, and thereby the size of the molten region 72, can be determined by the heat capacity of the layers of the disk 50. The temperature reached by the top layer of the disk 50, when exposed to a laser beam, is dependent upon three fundamental factors: the amount of energy per unit area that is introduced; the rate at which the energy is introduced; and the heat capacity and heat conductivity of the underlying material. The greater the thermal conductivity (or lower thermal resistance) of the underlying layer, the lower the temperature will be at the top layer. Conversely, the lower the thermal conductivity (or higher the thermal resistance) of the underlying material, the higher the temperature will be on the top surface.

In accordance with principles of the invention, a laser beam 70 having a short duration, on the order of 0.5 to 5 nanoseconds, for example, is used to melt the gold film 54 and underlying semiconductor layer 56. For a short duration of a laser beam 70, the heating rate is determined primarily by the energy supplied by the laser beam 70, as discussed below. The heat capacity of the disk 50 can be modeled using an equivalent electrical circuit model. Approximation of thermal models with electrical circuits is well known, and is not explained in detail herein.

An exemplary equivalent electrical circuit 80 of the disk 50 is shown in FIG. 7. On the left side of the dashed line 84 is the electrical circuit representation 82 of the semiconductor layer 56. To the right of the dashed line 84, the dielectric layer 58 used in the preferred embodiment is represented by a series of electrical components, and a unit length of the dielectric layer—1 micron for example—is represented by electrical circuit 86. In the equivalent electrical circuit 80, temperature is analogous to voltage, and the rate of change in temperature, or heat flux, is analogous to current. The heat dissipation is dependent upon the thermal conductivity of each layer of the disk 50.

If the heat dissipation for each layer on a disk 50 is known, the amount of heat required to raise the temperature of the gold film 54 and the underlying semiconductor layer 56 to the point where the gold film 54 and the semiconductor layer 56 will melt can be determined. If a short duration laser beam 70 is applied to the disk 50, the temperature change will be determined mostly by the heat capacity of the molten region 72, in which the energy of the laser beam 70 is absorbed. An equivalent circuit representation can be made for the total amount of heat transferred to the molten region 72. The total amount of heat transferred into the molten region 72 can be represented as follows: C=C_(p) ρ s²t units: J/°K  (1)

In equation (1), C is the total amount of heat transferred, C_(p) is the heat capacity of the semiconductor layer 56, ρ is the density of the semiconductor layer 56, s² is the surface area of the spot irradiated by the laser beam 70, and t represents the thickness of the semiconductor layer 56. The units are Joules (J) per degree Kelvin (K). This equation can be used to estimate the energy of the laser beam 70 required to melt the thin gold film 54 and the underlying semiconductor layer 56, and create the molten region 72 of the gold and semiconductor materials.

In an exemplary embodiment, a 1 nano-second long 0.65 milli-Watt laser pulse, having a wavelength of approximately 405 nm to 480 nm, may be used to irradiate the gold film 54. Such a laser pulse delivers an energy, ΔE, of 650 femtojoules to the surface of the gold film 54 to create the molten region 72. If the size of the molten region 72 is about 0.1 μm³, and assuming that the semiconductor layer 56 is a silicon layer having a heat capacity of 1.63 J/° K cm³, the temperature change in the gold film 54 and underlying semiconductor material 56 can be represented by the following equation: ΔT=ΔE/(C _(p) ρ s ² t)=400° C.  (2)

In FIG. 8, the temperature ramp up of the gold film 54 and underlying silicon semiconductor layer 56, which turn into the molten region 72, is shown. Using the parameters discussed above with respect to equation (2), the molten region 72 reaches over 400° C. after about 1 nanosecond. At 400° C., the molten region 72 has been heated above its eutectic temperature by the laser beam 70. With reference to the phase diagram of gold-silicon shown in FIG. 9, the eutectic temperature for a gold (Au) and silicon (Si) mixture is 363° C. The eutectic temperature is the lowest possible melting point of a mixture. In a eutectic reaction, a liquid is cooled down and separated into two materials. If germanium is used as the semiconductor layer 56, the eutectic temperature for a gold (Au) and germanium (Ge) mixture is 361° C. The phase diagram of a gold-germanium mixture shown in FIG. 10.

The time duration of the laser pulse can be adjusted to increase the beam's penetration of the gold film 54 into the underlying semiconductor layer 56. The time duration of the laser pulse may be increased, for example, if the amount of energy delivered by the laser pulse is insufficient to melt the gold film 54. If the amount of energy is more than sufficient to the melt the gold film 54, then the duration of the laser pulse may be reduced to increase the speed of the writing process, i.e., the laser pulse may be applied to a different location on the disk 50 in a quicker manner. Lower laser powers may require longer temperature ramp-up times.

Cooling of the molten region 72 is now discussed with reference to FIG. 4. Cooling of the molten region 72 is accomplished primarily via heat dissipation into layers of the disk 50 that are underneath the gold film 54. Both the heat capacity and the thermal conductivity of the materials underneath the molten region 72 are determining factors of the rate of cooling. In a preferred embodiment, the semiconductor layer 56 is formed over dielectric layer 58 comprising a silicon dioxide layer which has moderate heat capacity and high thermal resistance. The heat capacity of the oxide layer 58 is about the same as that of the semiconductor layer 56, if it is a silicon or germanium semiconductor layer. But, the thermal conductivity of the oxide layer 58 is about one hundred times lower than the semiconductor layer 56, if it is a silicon or germanium semiconductor layer 56.

The rate of cooling, or quenching, of the molten region 72 can be determined by an equivalent electrical circuit representation of heat conduction, as shown in FIGS. 11-13. FIG. 11 shows an equivalent circuit 120 having a series of electrical components 122 representing the thermal properties of the dielectric layer 58. In the equivalent electrical circuit 120, temperature is analogous to voltage and heat flux analogous to current. In FIG. 12, a wedge 90 is shown, which is a slice of the dielectric layer 58 having a depth d and a cross-sectional area A. The wedge 90 terminates at a heat sink 91. The time invariant steady state solution for the wedge 90 of the dielectric layer 58, terminated by a heat sink 91 with an infinite heat capacity, is a linear variation in temperature, where the thermal resistance of the sample is: R=d/(K A) units: ° K/W  (3)

In the above equation, d is the depth of the wedge 90 (as seen in FIG. 12), K is the heat conductivity of the dielectric layer 58, and A is the cross-sectional area of the wedge 90. For a silicon dioxide dielectric layer 58, the heat conductivity K=0.014 J/(sec·cm·° K). The rate of cooling is then determined by the R·C time constant of the equivalent electrical circuit. The R·C time constant is, generally, the time required for half of the heat to dissipate, or, in terms of the present invention, the time required for the ΔT to be reduced by 50%.

FIG. 13 shows a portion of the equivalent electrical circuit 120 of FIG. 11. In the portion of the equivalent electrical circuit shown in FIG. 13, the numeral 92 represents a 0.1 μm segment of the dielectric layer 58. Each 0.1 μm segment 92 of the dielectric layer 58 is represented by its thermal resistance R and heat capacity C_(o). FIG. 13 also shows the heat capacity C_(s) of the semiconductor layer 56. In a simple model, the resistance R of the first segment of the oxide layer is 7×10⁷° K/W, and its thermal capacity is 1.6×10⁻¹⁵ J/° K. According to these parameters, the R·C time constant is (7×10⁷° K/W) (1.6×10⁻¹⁵ J/° K), or approximately 100 nanoseconds. Thus, the exemplary embodiment, AT will be reduced by 50%—from 400° C. to 200° C., in 100 nanoseconds. As discussed above with reference to FIGS. 9 and 10, at 200° C., a mixture of gold and silicon or gold and germanium will be below their respective eutectic temperatures where solidification begins.

FIG. 14 is a graph showing the rate of cooling after the laser pulse is no longer irradiated onto the molten region 72. As discussed above, the time constant R·C of cooling in the equivalent electrical circuit simulation is approximately 100 nanoseconds. It has been found that such rapid cooling may result in the molten region 72 forming a meta-stable mixture, which is discussed below.

Before energy from a laser is directed toward the disk 50, the gold film 54 and the semiconductor layer 56 are in an equilibrium state. That is, the gold film 54 and semiconductor layer 56 are solid materials that are in contact with, but separate from, each other. However, in the non-equilibrium state, meta-stable transition phases of a molten material of the gold film 54 and semiconductor layer 56 have been found to exist. A thorough discussion of meta-stability is beyond the scope of this specification. Generally, meta-stability is the ability of a non-equilibrium state to persist for a long period of time. Many phases are meta-stable at room temperature for a very long period of time, but will rapidly decay at higher temperatures. For example, although a diamond is not stable at atmospheric pressures and should transform to graphite, at normal temperatures the transformation from diamond to graphite is extremely slow. If heated, however, the rate of transformation would increase and the diamond would become graphite.

Meta-stability can be caused by a relatively slow phase transformation, or, as in the case of the invention, can be caused by rapidly cooling a small portion of a molten mixture of the gold film 54 and semiconductor layer 56. There are three possible outcomes of rapidly cooling the molten region 72: all of the material goes into a meta-stable phase; some of the material goes into a meta—stable phase; or the mixture reaches an equilibrium state. For purposes of the invention, it does not make a difference what the outcome is. This is because, regardless of the final phase characteristics of the molten region 72, the top surface 74 will have desirable reflectance characteristics which is distinguishable from those of the gold film 54. Moreover, the meta-stable phase of the gold film 54 and the semiconductor layer 56 will be stable at the temperatures contemplated in the invention. Even if an equilibrium state is reached, the invention will provide desirable results because the fraction of gold film 54 which will be at the surface 74 of the molten region 72 will be significantly less than 50%.

With reference to FIGS. 15 and 16, a read operation for the disk 50 is now be described. As discussed above, heating above the eutectic temperature causes the formation of a molten mixture comprising the gold film 54 and the semiconductor layer 56. Upon cooling or quenching, the gold film 54 and the semiconductor layer 56 separate into a two component solid material.

After being melted and cooled, the gold film 54 no longer comprises the entirety of the upper surface of the molten region 72. As a result, there is a large change in the reflectance between the areas irradiated by the laser beam 70, and other areas where the gold film 54 was not heated by the laser beam. In FIGS. 15 and 16, numeral 100 represents molten region 72 after it has been cooled or quenched. The cooled region 100 comprises gold material 102 and semiconductor material 104. FIGS. 15 and 16 also show areas 106 where the gold film 54, unexposed to the laser beam during the write operation discussed above, has not been melted and is still mirror-like.

Data is read by observing the reflection at the surface of a disk of a low power laser beam, on the order of 100 micro-Watts or less, or approximately one third of the write power, for example. Where the gold film 54 has diffused into the underlying semiconductor layer 56, the reflectance of the laser beam is reduced due to significant absorption or scattering of the laser beam on the top surface 108 of the cooled region 100 (FIGS. 15 and 16). This is because the top surface 108 is now comprised, in substantial part, of semiconductor material 104. During a read operation, the high reflectance of areas 106 of gold film 54 is easily distinguished from the low reflectance of the top surface 108 of the cooled region 100.

A low power red laser having a wavelength of 650 nm—a wavelength where the reflectivity of gold is very high—can be used. The blue (or blue-violet) laser used for the write operation can also be used for the read operation, but at a lower power, on the order of 100 micro-Watts or less, or one third of the write power, for example. While the reflectivity of gold is not high at the wavelength of a low power blue laser, the read operation needs only to distinguish between the presence and absence of gold and can be accomplished using blue laser light.

FIG. 17 shows a system 150 for reading and writing data to disk 50 of the invention. The system 150 may comprise a central unit 152, a visual display 154 and a user interface 156. The disk 50 may be inserted into a slot 158 of the central unit 152. Inside the central unit 152 is a device for reading and writing data to the disk 50 in accordance with the invention. An example of such a device, generally designated by numeral 170, is shown in FIG. 18. The device 170 has a device controller 172, a processor 174, an optical controller 178 and an optical pickup 180. The processor 174 transmits and receives data which has been read from the disk 50 through the optical pickup 180 and data which will be written to the disk 50 through the optical pickup 180. The optical controller 178 controls the optical pickup 180. The device controller 172 controls the overall operation of the device 170. The device 170 also has a motor 182 for rotating a spindle 184 and the disk 50. The device 170 is capable of irradiating the disk 50 with a laser beam to melt the gold film 54 and underlying semiconductor layer 56 in order to write data to the disk 50, as discussed above. After data is written to disk 50, the device 170 can also read data from the disk 50 by irradiating a low power laser beam onto the disk 50.

The above discussed embodiments provide desirable results for long-term stability of archival data storage, and improve the maximum density of the recorded data. Archival storage requires long term stability of the materials, and gold is one of the least reactive materials known to mankind. The lifetime of a gold film and a semiconductor layer between two dielectric oxide layers should be essentially infinite. To the outside world the archival memory will look like a gold layer full of sub-micron size holes that are not very reflective. The hole/non-hole areas in a track represent data.

While the invention has been described in detail in connection with exemplary embodiments known at the time, it should be readily understood that the invention is not limited to such disclosed embodiments. Rather, the invention can be modified to incorporate any number of variations, alterations, substitutions or equivalent arrangements not heretofore described, but which are commensurate with the spirit and scope of the invention. For example, the oxide layers may be replaced with glass layers, and the invention can be used with lasers of different wavelengths that expose smaller areas of the gold film. Accordingly, the invention is not to be seen as limited by the foregoing description, but is only limited by the scope of the appended claims. 

1. A data storage medium comprising: a first material layer; and a second light reflective material layer formed over said first material layer and having a first light reflectance property, said first and second material layers having a property that a light beam applied to a region of said second material layer heats said first and second material layers at said region to cause a mixture of materials from said first and second material layers and a second light reflectance property for said region which is different from said first light reflectance property.
 2. The data storage medium of claim 1, wherein said second light reflective material layer is a gold containing film formed to a thickness of about 50 Å to about 300 Å.
 3. The data storage medium of claim 1, wherein said first material layer is a semiconductor layer formed to a thickness of about 200 Å to about 2000 Å.
 4. The data storage medium of claim 1, wherein said first light reflectance property is higher than said second light reflectance property.
 5. The data storage medium of claim 1, further comprising a dielectric layer under said first material layer.
 6. The data storage medium of claim 1, further comprising a protective layer over said second light reflective material layer.
 7. The data storage medium of claim 6, wherein said protective layer comprises a silicon dioxide layer formed to a thickness of about 300 Å to about 1000 Å.
 8. The data storage medium of claim 1, further comprising a substrate over said second light reflective material layer.
 9. The data storage medium of claim 1, wherein said first material layer comprises a silicon layer.
 10. The data storage medium of claim 1, wherein said first material layer comprises a silicon material and said second light reflective material layer comprises a gold material, and said mixture of materials comprises more of said silicon material than said gold material.
 11. The data storage medium of claim 1, wherein said first material layer comprises a germanium layer.
 12. The data storage medium of claim 1, wherein said first material layer comprises a germanium material and said second light reflective material layer comprises a gold material, and said mixture of materials comprises more of said germanium material than said gold material.
 13. The data storage medium of claim 1, wherein said mixture of materials is in a meta-stable phase.
 14. The data storage medium of claim 1, wherein said medium is a disk.
 15. A data storage medium comprising: a first dielectric layer; a semiconductor layer formed over said first dielectric layer; a gold containing film formed over said semiconductor layer; and a mixed material portion in said gold containing film and said semiconductor layer, said mixed material portion comprising a mixture of gold material and semiconductor material.
 16. The data storage medium of claim 15, wherein said gold containing film is formed to a thickness of about 50 Å to about 300 Å.
 17. The data storage medium of claim 15, wherein said semiconductor layer is formed to a thickness of about 200 Å to about 2000 Å.
 18. The data storage medium of claim 15, wherein said mixed material portion has a first reflectance value and said gold containing film has a second reflectance value higher than said first reflectance value.
 19. The data storage medium of claim 15, wherein said first dielectric layer has a first thermal conductivity and said semiconductor layer has a second thermal conductivity higher than said first thermal conductivity.
 20. The data storage medium of claim 15, wherein said first dielectric layer comprises a silicon dioxide layer.
 21. The data storage medium of claim 15, further comprising a second dielectric layer over said gold containing film.
 22. The data storage medium of claim 21, wherein said second dielectric layer comprises a silicon dioxide layer.
 23. The data storage medium of claim 15, further comprising a substrate over said gold containing film.
 24. The data storage medium of claim 15, wherein said semiconductor layer comprises a silicon layer.
 25. The data storage medium of claim 15, wherein said semiconductor material comprises a silicon material, and said mixed material portion comprises more of said silicon material than said gold material.
 26. The data storage medium of claim 15, wherein said semiconductor layer comprises a germanium layer.
 27. The data storage medium of claim 15, wherein said semiconductor material comprises a germanium material, and said mixed material portion comprises more of said germanium material than said gold material.
 28. The data storage medium of claim 15, wherein said mixed material portion is in a meta-stable phase.
 29. The data storage medium of claim 15, wherein said medium is a disk.
 30. A data storage medium, comprising: a high reflectance layer comprising a gold film formed over a semiconductor layer; and a plurality of low reflectance portions comprising a mixture of a gold material and a semiconductor material.
 31. The data storage medium according to claim 30, wherein said plurality of low reflectance portions have top surfaces comprising more semiconductor material than said gold material.
 32. The data storage medium according to claim 30, wherein said semiconductor material comprises a silicon material.
 33. The data storage medium according to claim 30, wherein said semiconductor material comprises a germanium material.
 34. The data storage medium according to claim 30, wherein said semiconductor layer is a silicon layer.
 35. The data storage medium according to claim 30, wherein said semiconductor layer is a germanium layer.
 36. The data storage medium according to claim 30, wherein said high reflectance layer is between about 50 Å to about 300 Å thick.
 37. The data storage medium according to claim 30, further comprising a dielectric layer underneath said semiconductor layer and another dielectric layer over said high reflectance layer.
 38. The data storage medium according to claim 30, further comprising a dielectric layer underneath said semiconductor layer and a substrate over said high reflectance layer.
 39. The data storage medium according to claim 30, wherein said medium is a disk.
 40. A data storage medium comprising: a first dielectric layer; a semiconductor layer formed over said first dielectric layer; and a gold containing film formed over said semiconductor layer.
 41. The data storage medium of claim 40, wherein said gold containing film is formed to a thickness of about 50 Å to about 300 Å.
 42. The data storage medium of claim 40, wherein said semiconductor layer is formed to a thickness of about 200 Å to about 2000 Å.
 43. The data storage medium of claim 40, wherein said first dielectric layer has a first thermal conductivity and said semiconductor layer has a second thermal conductivity higher than said first thermal conductivity.
 44. The data storage medium of claim 40, wherein said first dielectric layer comprises a silicon dioxide layer formed to a thickness of about 300 Å to about 1000 Å.
 45. The data storage medium of claim 40, further comprising a second dielectric layer over said gold film.
 46. The data storage medium of claim 45, wherein said second dielectric layer comprises a silicon dioxide layer.
 47. The data storage medium of claim 40, further comprising a polycarbonate substrate over said gold film.
 48. The data storage medium of claim 40, wherein said semiconductor layer comprises a silicon layer.
 49. The data storage medium of claim 40, wherein said semiconductor layer comprises a germanium layer.
 50. The data storage medium of claim 40, wherein said medium is a disk.
 51. A system for writing data to a recording medium, comprising: an optical recording medium comprising a first dielectric layer, a semiconductor layer formed over said first dielectric layer, and a gold containing film formed over said semiconductor layer; and a device capable of irradiating a laser beam onto the medium, and producing regions in said medium containing a mixture of material from said gold containing film and said semiconductor layer.
 52. The system of claim 51, wherein said laser beam is capable of melting said gold layer and said semiconductor layer.
 53. The system of claim 51, wherein said laser beam is capable of reading written data from said medium.
 54. The system of claim 51, wherein said device is capable of detecting a difference in reflectance of said laser beam between said gold containing film and an area containing a mixture of said semiconductor layer and said gold containing film.
 55. The system of claim 51, wherein said medium is a disk.
 56. A method of writing data to a medium, comprising: providing a disk having a dielectric layer, a semiconductor layer over said dielectric layer, and a gold film over said semiconductor layer having a predetermined reflectivity; and irradiating a laser light beam onto said gold film with sufficient energy to diffuse said gold film into said semiconductor layer to create an area having a reflectivity lower than said predetermined reflectivity.
 57. The method of claim 56, wherein said semiconductor layer comprises a silicon semiconductor layer, and said act of irradiating said laser light beam comprises raising a temperature of said gold film and said silicon semiconductor layer above a eutectic temperature for a gold-silicon mixture.
 58. The method of claim 56, wherein said semiconductor layer comprises a germanium semiconductor layer, and said act of irradiating said laser light beam comprises raising a temperature of said gold film and said germanium semiconductor layer above a eutectic temperature for a gold-germanium mixture.
 59. The method of claim 56, wherein said gold film has a thickness of between about 50 Å to about 300 Å.
 60. The method of claim 56, wherein said semiconductor layer has a thickness of between about 200 Å to about 2000 Å.
 61. The method of claim 56, wherein said dielectric layer is a silicon dioxide layer having a thickness of between about 300 Å to about 1000 Å.
 62. The method of claim 56, wherein said act of irradiating said laser light beam comprises raising a temperature of said gold film to at least 400° C.
 63. The method of claim 56, wherein said act of irradiating said laser light beam comprises irradiating a light beam having a wavelength in the range of 405 nm and 480 nm.
 64. The method of claim 56, further comprising cooling said gold film and said semiconductor layer after said act of irradiating.
 65. The method of claim 64, wherein said act of cooling comprises cooling through dissipation of heat into material layers underneath said semiconductor layer.
 66. The method of claim 65, wherein said material layers underneath said semiconductor layer dissipate said heat such that a temperature of said gold film and said semiconductor layer is reduced to a temperature below a eutectic temperature in no longer than approximately 100 nanoseconds.
 67. The method of claim 56 wherein said medium is a disk.
 68. The method of claim 56 wherein said irradiating a laser light beam comprises irradiation a blue light laser beam.
 69. A method of changing reflectance on a data storage medium, comprising: irradiating a laser light beam onto a gold film formed over a semiconductor layer to raise a temperature of said gold film above a eutectic temperature for a mixture of gold and said semiconductor layer, thereby creating a mixture of material from said gold film and material from said semiconductor layer; and cooling said mixture; whereby said irradiating and cooling acts to produce a top surface on said cooled mixture containing more of said material from said semiconductor layer than said material from said gold film.
 70. The method of claim 69, wherein said irradiating said laser light beam comprises irradiating a laser beam having a wave length in the range of approximately 405 nm to approximately 480 nm.
 71. The method of claim 69, wherein said semiconductor layer comprises a silicon layer.
 72. The method of claim 69, wherein said semiconductor layer comprises a germanium layer.
 73. The method of claim 69, wherein said data storage medium is a disk.
 74. The method of claim 69, wherein said irradiating a laser light beam comprises irradiating a blue laser light beam. 